1. Field of the Invention
The present invention relates to an engine exhaust gas recirculation system effective to reduce the emission of nitrogen oxides (NO.sub.x).
2. Description of the Prior Art
The prior art engine exhaust gas recirculation systems (hereunder termed "E.G.R. system") conventionally used to reduce NO.sub.x emission are classified into plate type and manifold type. The E.G.R. system of the plate type includes an E.G.R. passage extending from an exhaust pipe of an engine to an intake pipe upstream of a throttle valve. A fixed orifice is provided in the E.G.R. passage. The E.G.R. system of the manifold type has an E.G.R. passage extending from the exhaust pipe to the intake pipe downstream of the throttle valve. A valve is provided in the E.G.R. passage to control the recirculation of engine exhaust gases back into the intake pipe.
In the first or plate type E.G.R. system, because engine exhaust gases flow back into the intake pipe upstream of the throttle valve, the engine back pressure, which is a function of the amount of engine intake air, is solely the function of the amount of exhaust gas recirculation. The amount of the recirculated exhaust gases is thus advantageously proportional to the amount of the engine intake air. The E.G.R. system of this type, however, has disadvantageous problems that a deposit of a foreign material is formed on the throttle valve, an advance port is blocked, the carburetor and related components of the engine suffer from thermal influence, parts made of aluminum or aluminum alloys are corroded and icing occurs at a low temperature.
In the E.G.R. system of the second or manifold type, the above disadvantageous problems hardly occur because the exhaust gases are recirculated directly into the engine intake manifold downstream of the throttle valve. In the second type of E.G.R. system, however, the recirculation of exhaust gases is influenced by the engine intake manifold vacuum and thus is increased and decreased in light and heavy operating conditions of engine, respectively. Thus, valve means are required for this type of E.G.R. system to control the recirculation of exhaust gases such that the exhaust gas recirculation is not influenced by the engine intake manifold vacuum. In a conventional E.G.R. system, the E.G.R. control valve is simply controlled by the intake manifold vacuum such that the valve is closed to interrupt the recirculation of the exhaust gases at an engine operating condition where the exhaust gas recirculation is undesirable to the engine operation. The intake manifold vacuum is formed into ON and OFF signals depending upon the positions of the throttle valve. In another conventional E.G.R. system, either intake manifold vacuum or venturi vacuum is used to control the E.G.R. control valve so that the valve opening is decreased at light load engine operating condition to prevent the exhaust gas recirculation from being influenced by the intake manifold vacuum and so that the valve opening is increased to increase the exhaust gas recirculation at heavy load engine operating condition. In any case, however, the influence of the intake vacuum on the exhaust gas recirculation could not completely be eliminated to such an extent where the exhaust gas recirculation was made proportional to the engine intake air. In the prior art E.G.R. system, therefore, the exhaust gas recirculation was unduely increased relative to the intake air flow at a light load engine operating condition with resultant occurence of surging and misfires, whereas the exhaust gas recirculation was decreased relative to the intake air flow at a heavy load engine operating condition with resultant decrease in the reduction of NO.sub.x emission. With the prior art E.G.R. system, therefore, it has been difficult to reduce NO.sub.x emission without adverse affect on the engine operation.
In applicants' earlier copending application Ser. No. 756,702 referred to above, a manifold type exhaust gas recirculation system is disclosed which comprises an E.G.R. passage for the recirculation of engine exhaust gases from an exhaust system of an engine back into an intake system thereof downstream of an engine throttle valve. A first E.G.R. control valve is provided at a first point in the E.G.R. passage and operatively connected to the throttle valve to control E.G.R. flow through the E.G.R. passage at the first point. A second E.G.R. control valve is provided at a second point in the E.G.R. passage upstream of the first E.G.R. control valve to control E.G.R. flow through the E.G.R. passage at the second point toward the first point. A valve controller is provided for the second E.G.R. control valve and operative in response to the variation in the exhaust gas pressure in the E.G.R. passage between the first and second E.G.R. control valves to control the second E.G.R. control valve so that the exhaust gas pressure between the first and second E.G.R. control valves is maintained substantially constant.